The present invention relates to a piezoelectric film laminate and a method of manufacturing the same.
A demand for a surface acoustic wave device has rapidly increased along with a remarkable development in the communication field represented by mobile communication utilizing a portable telephone or the like. The development of the surface acoustic wave device is trending toward a reduction in size and an increase in efficiency and frequency. This requires a higher electromechanical coupling factor (coefficient) (k2), more stable temperature properties, and a higher surface acoustic wave propagation velocity. When using a surface acoustic wave device as a high frequency filter, a high electromechanical coupling factor is desired in order to obtain a passband with a small loss and a wide bandwidth. In order to increase the resonance frequency, a material exhibiting a higher speed of sound is desired from the viewpoint of limitations to the design rule for the pitch of interdigital transducers. In order to obtain stable properties at an operating temperature, the temperature coefficient of frequency (TCF) must be small.
A surface acoustic wave device having a structure in which interdigital transducers are formed on a piezoelectric single crystal has been mainly used. As typical examples of the piezoelectric single crystal, a rock crystal, lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and the like can be given. For example, LiNbO3 having a high electromechanical coupling factor is used for an RF filter for which an increase in band and a decrease in loss in the passband are required. A rock crystal having a small temperature coefficient of frequency is used for an IF filter for which stable temperature properties are required in a narrow band. LiTaO3 having an intermediate electromechanical coupling factor and temperature coefficient of frequency between those of LiNbO3 and a rock crystal plays an intermediate role between LiNbO3 and a rock crystal. However, the electromechanical coupling factor of LiNbO3 is only about 20%.
In recent years, a cut angle of a potassium niobate (KNbO3) (a=0.5695 nm, b=0.5721 nm, c=0.3973 nm; this index indication is hereinafter used for an orthorhombic crystal) single crystal showing a high electromechanical coupling factor has been found. As described in Eletron. Lett. Vol. 33 (1997) 193., it was calculated that a 0°Y-cut X-propagation (“0°Y-X”) KNbO3 single crystal plate shows an electromechanical coupling factor as high as 53%. As described in Jpn. J. Appl. Phys. Vol. 37 (1998) 2929., it was confirmed by experiments that a 0° Y-X KNbO3 single crystal plate shows an electromechanical coupling factor as high as 50%, and the oscillation frequency of a filter using a 45° to 75° rotated Y-X KNbO3 single crystal substrate shows zero temperature properties at or near room temperature. JP-A-10-65488 discloses rotated Y-X KNbO3 single crystal plates including a 0° Y-X KNbO3 single crystal plate.
In a surface acoustic wave device using a piezoelectric single crystal substrate, properties such as the electromechanical coupling factor, temperature coefficient, and speed of sound are values specific to the material and are determined by the cut angle and the propagation direction. A 0° Y-X KNbO3 single crystal substrate has a high electromechanical coupling factor, but does not show zero temperature properties at or near room temperature, differing from a 45° to 75° rotated Y-X KNbO3 single crystal substrate. A 0° Y-X KNbO3 single crystal substrate shows a propagation velocity lower than that of SrTiO3 or CaTiO3 which is also a perovskite type oxide. As described above, a high speed of sound, a high electromechanical coupling factor, and zero temperature properties cannot be obtained by merely using a KNbO3 single crystal substrate.
The speed of sound, electromechanical coupling factor, and temperature properties are expected to be increased by depositing a piezoelectric thin film on a substrate and controlling the thickness of the piezoelectric thin film. Jpn. J. Appl. Phys. Vol. 32 (1993) 2337. discloses a zinc oxide (ZnO) thin film formed on a sapphire substrate, and Jpn. J. Appl. Phys. Vol. 32 (1993) L745. discloses a LiNbO3 thin film formed on a sapphire substrate, for example. Therefore, the above-mentioned properties are expected to be improved by forming a KNbO3 thin film on a substrate.
It is desirable that the piezoelectric thin film be oriented in an optimum direction so that the electromechanical coupling factor and the temperature properties are improved. In order to reduce a loss caused by leaky wave propagation, it is desirable that the piezoelectric thin film be a flat and dense epitaxial film. Y-X KNbO3 having an electromechanical coupling factor of 50% corresponds to a pseudocubic (100) crystal, and 90°Y-X KNbO3 having an electromechanical coupling factor of 10% corresponds to a pseudocubic (110) crystal. Therefore, a Y-X KNbO3 thin film having an electromechanical coupling factor of 50% or a 90°Y-X KNbO3 thin film having an electromechanical coupling factor of 10% is expected to be obtained by using an SrTiO3 (100) or (110) single crystal substrate.
However, a single phase potassium niobate thin film and a Y-X epitaxial potassium niobate thin film have not yet been formed on a large insulator substrate.